How to test the strength of metal 3D printing molds?

一, The main method for strength testing
Strength testing of metal 3D printing molds must encompass three dimensions: material attributes, microstructure, and defect control, creating a multi-level and multi-dimensional testing loop.
1. Testing the mechanical performance: Index of quantitative strength
Tensile test: Use a universal testing equipment to put an axial tensile load on the mold material. This will tell you its tensile strength (UTS), yield strength (YS), and elongation at break (EL). For instance, titanium alloy molds must fulfill the ASTM E8 standard, and their UTS must be between 800 and 1000 MPa, while their YS must be at least 700 MPa.
Compression test: Check how stable the mold is when it is compressed. This test is especially useful for supporting structures or components with thick walls. For instance, the GB/T 7314 standard needs to be used to check the compressive strength of aluminum alloy molds to make sure they don't bend when they are formed under high pressure.
The three-point or four-point bending test is performed to see how rigid the mold is as it bends and how strong the bond between the layers is. Testing the bending strength of stainless steel molds according to ISO 14125 is one way to keep interlayer peeling from causing failure.
Impact test: Use a pendulum impact testing machine to mimic dynamic loads and find out how resistant the mold is (for example, the Charpy V-notch impact energy). For instance, the mold for aviation engine blades must meet the ASTM E23 standard to make sure it doesn't break easily when used in harsh situations.
Fatigue test: Use a high-frequency fatigue testing machine to simulate cycle loads and figure out how long the mold will last before it gets tired. Automotive molds, for instance, must fulfill the ISO 12107 standard and have a fatigue limit of at least 300 MPa to handle the stresses of long-term high-pressure stamping.
2. Microstructure analysis: finding out what makes strength possible
Use a metallographic microscope (OM) to look at the mold's grain size, phase composition, and interlayer interface. For instance, molds made with SLM (Selective Laser Melting) often have microscopic equiaxed grains, which make them more than 30% stronger than standard castings.
Scanning electron microscope (SEM): Look at how cracks start and spread, and look for flaws like lack of fusion and porosity. For instance, SEM needs to check EBM (electron beam melting) molds to make sure they are not too porous (≤ 0.5%) and to avoid stress concentration.
Electron backscatter diffraction (EBSD): Measure the difference in crystal orientation (KAM value) and test how well the material can deform locally. For instance, places with high KAM values are likely to develop cracks, and heat treatment is needed to improve the orientation of the grains.
3. Find defects: get rid of hidden strength risks
CT scanning for industry: Testing for internal flaws in molds that don't damage them, like pores, fissures, and spots where the mold hasn't fully fused. For instance, aviation molds must meet the ISO 17637 standard to make sure that defects are no bigger than 0.1mm and that fatigue cracks don't spread.
Ultrasonic testing (UT) finds faults that are deep inside, and it works best on molds with thick walls. For instance, car panel molds must meet the ASTM E233 standard in order to find interior flaws that are at least 50 mm deep.
X-ray fluorescence spectroscopy (XRF): Quickly check the mold's chemical makeup to make sure the element content is what it should be. Nickel-based alloy molds, for instance, must meet the ISO 3497 standard to keep the ranges of critical constituents like Cr and Mo below 0.5%.
二, System of testing standards for both international and domestic use
Metal 3D printing molds must be tested for strength according to strict international and domestic standards to make sure that the data can be compared and that the rules are followed.
1. Standards from throughout the world
ISO/ASTM 52900: Establishes definitions and classifications for 3D printing terminology, offering a foundational basis for strength testing.
ISO 23499: sets the standards for the dimensional accuracy and surface quality of metal 3D printed items. This has an indirect effect on strength testing.
ASTM E8/E23/E466: These are the main standards for mold strength testing. They tell you how to do tensile, impact, and fatigue tests.
2. Standards for the home
GB/T 39251: Make clear the rules for preparing and using samples for tensile, compressive, bending, and other tests of the mechanical properties of 3D printed metal materials.
GB/T 39651: lays out the steps for inspecting and evaluating metal additive manufacturing parts, including how to classify defects and set acceptance criteria.
QB/T 5696: This standard talks about the quality of metal 3D printing materials and sets particular requirements for things like the size and flowability of powder particles, which affect the strength of the mold in an indirect way.
三, Important technical problems and how to fix them
To make metal 3D printing molds stronger, we need to find ways to deal with problems like anisotropy, residual stress, and surface quality. This can be done through new technology and better processes.
1. Controlling anisotropy
The strength of 3D printed molds changes based on the printing orientation (longitudinal, transverse, oblique). For example, the longitudinal strength of SLM generated molds could be 20% higher than the transverse strength.
Solution: Improve the way you scan: Use a checkerboard or spiral scanning path to make the changes in thermal stress across layers less noticeable.
Post-processing strengthening: Hot isostatic pressing (HIP) removes pores, raising the mold's density to over 99.9%.
Multi-directional testing: Samples are taken in three different directions-longitudinal, transverse, and oblique-to make sure that the minimum strength is up to the design standards.
2. Managing residual stress Problem: When 3D printing cools too quickly, it can build up residual stress, which can cause the mold to bend or shatter.
Solution: Stress relief annealing: Keep it at 500-600 °C for 2 to 4 hours to let go of internal stress.
Laser shock peening uses high-energy laser beams to change the shape of the surface, add compressive residual stress, and make the material survive longer.
Online monitoring: Using built-in fiber optic sensors to keep an eye on stress distribution in real time and change printing settings as needed.
3. Better surface quality Problem: The surface roughness (Ra) of 3D printed molds is usually 10–20 μm, which makes them easy to shatter.
Answer:
Mechanical polishing: A CNC polishing machine may lower Ra to less than 0.8 μm, which makes the surface stronger.
Chemical polishing: employing acid washing or electrolytic polishing to get rid of small surface flaws and make the surface more resistant to corrosion.
Shot peening: This process adds a layer of residual compressive stress to the surface by hitting it with high-speed projectiles. This makes it more resistant to fatigue.
四, Case Study and Industry Practice
Case 1: Mold for the blade of an aircraft engine
Nickel-based high-temperature alloy (Inconel 718) is the material.
Focus of the test:
High temperature strength: The GB/T 4338 standard says that the tensile strength should be tested at 650 °C to make sure it is at least 800 MPa.
Thermal fatigue performance: Use ISO 12111 to test the rate of fracture propagation during engine start-stop cycles.
Microstructure: Use EBSD to look at the distribution of the γ 'phase and make sure that the size of the strengthening phase is ≤ 50 nm. This will make the material more stable at high temperatures.
The mold lasts three times longer than standard castings, which is what is needed for aircraft engines to run for 100,000 hours.
Case 2: Mold for an automotive cover
Material: Steel with a high strength (H13)
What to test:
Wear resistance: The ASTM G65 standard tests the wear quantity to make sure it is ≤ 0.1 g/1000 rotations.
Impact resistance: The Charpy impact energy must be at least 30 J, according to the ISO 148 standard.
Dimensional accuracy: Use CMM testing to make sure that the mold surface inaccuracy is less than or equal to 0.05 mm.
The mold lasts for 500,000 stamping cycles, which is 50% longer than other molds.